Redfin ( English Perch) are an introduced species that have become well established in many public and private waterways in NSW, Victoria, South Australia, Western Australia, Southern Queensland, the ACT and Tasmania. They can tolerate lowland freshwater at sea level near coastal towns, inland waterways and also higher mountain streams and lakes. There has been a lot of questions come to this column including many asking to explain the EHN scary virus that some redfin carry, so I got our leading freshwater fisheries biologist Simon Kaminskas on the Job:

"EHN is a virus that kills Redfin. EHN stands for Epizootic Haematopoietic Necrosis Virus - Epizootic meaning an illness affecting animals, Haematopoeitic meaning to do with organs of the blood, eg liver, kidneys, Necrosis meaning death of said organs, tissues.

EHN Virus has an interesting history. It first appeared in some area of NE Victoria - Eildon, and caused annual fish kills of juvenile Redfin
there. It has slowly spread since then and is now found in patches throughout the bottom half of Australia.

Some mind blowing research now suggests that it was not introduced from Europe or England, as first thought, but was an endemic virus that had been lying dormant for millenia because their host no longer existed, for whatever reason - UNTIL introduced Redfin came along and just happened to an organism it could make it's new host. Amazing but creepy!

EHN is very lethal, but almost only to Redfin. It, as the explanation of the name above suggests, kills Redfin by causing massive haemorraging of their kidney and liver. If you examine very small Redfin killed by EHN, because they're semi-translucent, you can see the blood inside their body cavity where they've been haemorraging.

When EHN first got gets into a new waterway it causes MASSIVE fish kills. And there may not be a single Redfin caught for several years
afterwards.

The Redfin populations then fluctuate. After the first massive walloping, they slowly start to recover, and just as they were get half-common again, EHN knocks them flat. They start to recover, EHN knocks them flat. Etc, etc, etc. This has been happening for about a decade now.

Now, finally, Redfin have made a strong comeback in previously affected areaswithout EHN hitting them. This has had one clear effect - creating some year classes of very big Redfin that grew large in previous years when there were little other Redfin to compete with for food. I believe I have proven this by catching Redfin to 45cm, and regularly catching them in excess of 30cm; fishing I have never had before."

Thanks for the detailed explanation Simon. Redfin are a introduced species that have caused damage to native fish populations. Even so they are generally now accepted as a sports fish because they are much of less concern than carp which compete with native fish, but even worse do a whole heap of damage to the environment by the way they suck up vegetation form lake and river beds.

The next big issue is the Federal government's idea of introducing viruses to eradicate carp. A good idea, but not without the dangers of the virus being transferable to native fish species. Simon will look at this fascinating topic a little down the track after the government releases its plans.